Adding multi-touch sensing to the surface of a mouse has the potentialto substantially increase the number of interactions availableto the user. However, harnessing this increased bandwidth ischallenging, since the user must perform multi-touch interactionswhile holding the device and using it as a regular mouse. In thispaper we describe the design challenges and formalize the designspace of multi-touch mice interactions. From our design spacecategories we synthesize four interaction models which enable theuse of both multi-touch and mouse interactions on the same device.We describe the results of a controlled user experiment evaluatingthe performance of these models in a 2D spatial manipulationtask typical of touch-based interfaces and compare them tointeracting directly on a multi-touch screen and with a regularmouse. We observed that our multi-touch mouse interactions wereoverall slower than the chosen baselines; however, techniquesproviding a single focus of interaction and explicit touch activationyielded better performance and higher preferences from ourparticipants. Our results expose the difficulties in designing multitouchmice interactions and define the problem space for futureresearch in making these devices effective.